Passive cooling relying on evaporation and radiation, while offering great energy-saving opportunities, faces challenges with low ambient cooling powers, environmental heating, high water usage, and climate condition constraints. To overcome these shortcomings, here, we present insulated cooling with evaporation and radiation (ICER), which utilizes a solar-reflecting layer, an infrared-emitting evaporative layer, and an infrared-transparent, solar-reflecting, and vapor-permeable insulation layer. ICER consistently achieved below-wet-bulb temperatures with much less water consumption than pure evaporation while reaching 9.3 Celsius below the ambient temperature under direct sunlight. With unfavorable climate conditions, ICER delivered 96 W/m² daytime cooling power at the ambient temperature and showed 300% enhancement over the state-of-the-art radiative cooler. During the summer months, without electricity, ICER can extend food shelf-life by 40% in humid climates and 200% in dry climates with low water-refilling frequencies.

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Cell Report Physical Science 2022, MIT News.

We present a surface-engineering approach that turns all liquids highly wetting, including ultra-high surface tension fluids such as mercury. Previously, highly wetting behavior was only possible for intrinsically wetting liquid/material combinations. Here, we show that roughness made of reentrant structures allows for a metastable hemiwicking state even for nonwetting liquids as predicted by our surface energy model. We experimentally demonstrated this concept with microfabricated reentrant channels. Notably, we show an apparent contact angle as low as 35° for mercury on structured silicon surfaces with fluorinated coatings, on which the intrinsic contact angle of mercury is 143°, turning a highly nonwetting liquid/material combination highly wetting through surface engineering. Our work enables highly wetting behavior for previously inaccessible material/liquid combinations and thus expands the design space for various thermofluidic applications.

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PNAS 2022, MIT News - Spotlight.

Passive thermal management strategies show promise to alleviate the ever-increasing global energy demand for cooling which is projected to triple in 2050. Passive cooling also provides viable pathways to distribution and storage of food and pharmaceuticals in underdeveloped areas, given that >10% of the world’s population still have no access to electricity. Due to the ease of implementation and potential high cooling powers, evaporative cooling has emerged as one of the most promising passive cooling solutions. However, the state-of-the-art evaporative cooling technologies are largely limited by environmental heating. Here, inspired by the fur layer of desert animals, we address this critical challenge with a transparent bilayer made of hydrogels and aerogels, which allows for evaporative cooling and cuts parasitic heat gain at the same time. Our bilayer cooling structure significantly can increase the effective cooling time by 400% compared to a conventional single layer design.

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Joule 2020, MIT News, Popular Mechanics, New Scientist, Smithsonian Magazine, E&E News, BBC Science Focus.

Bubble nucleation is ubiquitous in gas evolving reactions which are instrumental for a variety of electrochemical systems. Fundamental understanding of the nucleation process, which is critical to system optimization, remains limited as prior works generally focused on the thermodynamics and have not considered the coupling between surface geometries and different forms of transport in the electrolytes. Here, we establish a comprehensive transport-based model framework to identify the underlying mechanism for bubble nucleation on gas evolving electrodes. We identify the significance of the gas diffuse layer thickness, a parameter controlled by external flow fields and overall electrode geometries, which has been largely overlooked in previous models. Our model framework offers guidelines for practical electrochemical systems whereby without changing the surface chemistry, nucleation on electrodes can be tuned by engineering the cavity size and the gas diffuse layer thickness.

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Langmuir 2020

Water is often considered as the highest performance working fluid for liquid-vapor phase change due to its high thermal conductivity and large enthalpy of vaporization. However, a wide range of industrial systems requires using low surface tension liquids where heat transfer enhancement has proved challenging for boiling and evaporation. Here, we enable a new paradigm of phase change heat transfer, which favors high volatility, low surface tension liquids rather than water. We utilized a nanoporous membrane of about 600 nm thickness and less than 140 nm pore diameters supported on efficient liquid supply architectures, decoupling capillary pumping from viscous loss. Proof-of-concept devices were microfabricated and tested in a custom-built environmental chamber. R245fa, pentane, methanol, isopropyl alcohol, and water were used as working fluids. We then compared pore-level heat transfer of different fluids, where R245fa showed approximately 10 times the performance of water under the same working conditions. Finally, we illustrate the usefulness of a figure of merit extracted from kinetic theory for evaporation. The current work provides fundamental insights into evaporation of low surface tension liquids, which can impact various applications such as refrigeration and air conditioning, petroleum and solvent distillation, and on-chip electronics cooling.

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ACS Appl. Mater. Interfaces 2020

We experimentally realized and elucidated kinetically limited evaporation where the molecular gas dynamics close to the liquid-vapor interface dominate the overall transport. This process fundamentally dictates the performance of various evaporative systems and has received significant theoretical interest. However, experimental studies have been limited due to the difficulty of isolating the interfacial thermal resistance. Here, we overcame this challenge using an ultrathin nanoporous membrane in a pure vapor ambient. We showed that kinetically limited evaporation, when normalized properly, is solely determined by the pressure ratio between the ambient and the interface. We modeled the nonequilibrium gas kinetics and demonstrated good agreement between experiments and theory. Our work illustrates a unified fundamental relationship between the interfacial flux and the driving potential for evaporation in the dimensionless form. It also provides a general figure of merit for evaporative heat transfer as well as design guidelines for achieving efficient evaporation in applications such as water purification, steam generation, and thermal management.

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Nature Communications 2019

MIT News

Evaporation is a ubiquitous phenomenon found in nature and widely used in industry. Yet a fundamental understanding of interfacial transport during evaporation remains limited to date owing to the difficulty of characterizing the heat and mass transfer at the interface, especially at high heat fluxes (>100 W/cm²). In this work, we elucidated evaporation into an air ambient with an ultrathin (≈200 nm thick) nanoporous (≈130 nm pore diameter) membrane. With our evaporator design, we accurately monitored the temperature of the liquid–vapor interface, reduced the thermal–fluidic transport resistance, and mitigated the clogging risk associated with contamination. At a steady state, we demonstrated heat fluxes of ≈500 W/cm² across the interface over a total evaporation area of 0.20 mm². In the high flux regime, we showed the Maxwell-Stefan equation governs the flow instead of Fick’s first law of diffusion. This work improves our fundamental understanding of evaporation and paves the way for high flux phase-change devices.

Related article: Nano Letters 2017

Textured surfaces are instrumental in water repellency or fluid wicking applications, where the pinning and depinning of the liquid–gas interface plays an important role. We demonstrate that a contact line can exhibit nonuniform behavior even without varying the local energy barrier. Around a cylindrical pillar, an interface can reside in an intermediate state where segments of the contact line are pinned to the pillar top while the rest of the contact line moves along the sidewall. This partially pinned mode is due to the global nonaxisymmetric pattern of the surface features and exists for all textured surfaces, especially when superhydrophobic surfaces are about to be flooded or when capillary wicks are close to dryout.

Related article: Langmuir 2017

High power density electronics are severely limited by current thermal management solutions which are unable to dissipate the necessary heat flux while maintaining safe junction temperatures for reliable operation. We designed, fabricated, and experimentally characterized a microfluidic device for ultra-high heat flux dissipation using evaporation from a nanoporous silicon membrane. With ~100 nm diameter pores, the membrane can generate high capillary pressure even with low surface tension fluids. The suspended ultra-thin membrane structure facilitates efficient liquid transport with minimal viscous pressure losses. We microfabricated and experimentally characterized the devices in pure vapor-ambient conditions in an environmental chamber. Accordingly, we demonstrated heat fluxes of 665 ± 74 W/cm² using pentane over an area of 0.172 mm × 10 mm with a temperature rise of 28.5 ± 1.8 K from the heated substrate to ambient vapor.

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Microsystems & Nanoengineering 2018

IEEE Trans. Compon. Packag. Manuf. Technol. 2016

Evaporation from nanopores is of fundamental interest in nature and various industrial applications. We present a theoretical framework to elucidate evaporation and transport within nanopores by incorporating non-equilibrium effects due to the deviation from classical kinetic theory. Additionally, we include the non-local effects arising from phase-change in nanoporous geometries, and the self-regulation of the shape and position of the liquid-vapor interface in response to different operating conditions. We then study the effects of different working parameters to determine conditions suitable for maximizing evaporation from nanopores.

Related article: Langmuir 2015